CURLOPT_READDATA (3) - Linux Manuals
CURLOPT_READDATA: custom pointer passed to the read callback
NAMECURLOPT_READDATA - custom pointer passed to the read callback
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READDATA, void *pointer);
DESCRIPTIONData pointer to pass to the file read function. If you use the CURLOPT_READFUNCTION(3) option, this is the pointer you'll get as input in the 4th argument to the callback.
If you don't specify a read callback but instead rely on the default internal read function, this data must be a valid readable FILE * (cast to 'void *').
If you're using libcurl as a win32 DLL, you MUST use a CURLOPT_READFUNCTION(3) if you set this option.
DEFAULTBy default, this is a FILE * to stdin.
PROTOCOLSThis is used for all protocols when sending data.
AVAILABILITYThis option was once known by the older name CURLOPT_INFILE, the name CURLOPT_READDATA was introduced in 7.9.7.
RETURN VALUEThis will return CURLE_OK.
SEE ALSOCURLOPT_READFUNCTION(3), CURLOPT_WRITEDATA(3),